The present invention concerns methods and circuit arrangements for automatically monitoring functions of a radio receiver, especially the HF sensitivity and the LF output-signal level of such receivers, during pauses in reception, the method and circuit arrangements in question being of the type wherein a HF test signal is applied to the input of the HF stage of the receiver and the version of the test signal appearing at the output of the LF stage of the receiver is rectified and then evaluated with respect to proper functioning or malfunction of the receiver.
An automatic monitoring method of the type in question is described in Federal Republic of Germany patent DT-PS 1,466,552. In that method, use is made of a warbling test signal whose frequency is cyclically varied within a predetermined frequency range centered about the HF frequency value which the radio receiver is to receive. The difference between the frequency value which the receiver is to receive and either limit of the frequency range through which the test signal is warbled is made greater than the signal bandwidth which the receiver is capable of receiving. If the receiver is capable of receiving a plurality of different channels, then the test signal employed is cyclically varied in frequency within a frequency range which is greater than the frequency range of all channels receivable by the receiver. In that latter instance, it is unnecessary to switch over from one test signal to another when monitoring proper receiver functioning at different ones of its receivable frequencies.
In that system, the test signal is transmitted through the HF stage, the IF stage, and the LF stage of the receiver and then rectified. The rectified test signal controls the operation of a relay which, in the event of receiver malfunction, generates an alarm or other malfunction indication and/or automatically switches on a stand-by or emergency receiver. In principle, such a malfunction indication and/or automatic emergency switchover does occur, with such prior-art system, in response to excessively low HF sensitivity of the receiver and in response to an excessive drop in the LF output-signal level. However, with the prior-art technique, it is extremely difficult to separately preselect, e.g., completely independently of each other, the drop in HF sensitivity and the drop in LF output-signal level which will actually result in a malfunction indication or an automatic emergency switchover.
Federal Republic of Germany patent DT-PS 1,034,716 discloses an automatic monitoring circuit for FM receivers in which, however, proper functioning of the receiver is not monitored beyond the output of the receiver's IF stage. The test signal there employed is a low-frequency voltage with which the inherent noise of the HF input stage itself is amplitude modulated. The test signal is automatically applied to the input amplifier of the HF stage, but only when the signal level at the output of the IF stage falls below a predetermined value, and simultaneously therewith the receiver's antenna is automatically switched off or disconnected. This automatic test-signal application occurs, however, even during normal and malfunction-free reception, if it should happen that at a particular moment only a weak signal is being received, and the antenna is not automatically switched back on or into circuit with the remainder of the receiver until after the automatically triggered test-signal cycle has finished. Accordingly, the automatic monitoring system leads to undesirable breaks in reception. Furthermore, the voltage of the noise generated within the HF stage itself, positively exploited in that system, is not constant over long periods of operation and fluctuates in dependence upon various operating conditions. This makes it impossible to precisely preestablish quantitatively what levels of malfunction or poor performance are and are not to be permitted to trigger a malfunction indication.